I was running Isle of Thieves, first time there, with Storm Island -> The Swarm activated. This caused three Gargoyles, two Sand Devils, two Sunken Ones, and two Cave Bats to take the place of the termites outside of Slayne's cave. My route over the island typically took me about 15 minutes to run, each loop, netting about 55 or 56 kills each run. (First run was about mid-40s; second run on was in the 50s. Discovered an extra Giant Crab about halfway through.) It took me 15 full loops, plus about a quarter of a 16th to hit 2000 from whatever I had previously. In total, about 4 hours of play time.

This particular route had me fighting, in addition to the monster classes posted above, Giant Crabs, Termite Workers and Warriors, a Leviathan, and a Panther. The extra Giant Crab was found on the initial beach: it pops up out of the sand a bit southwest of the visible crab. The achievement popped upon killing a Termite Worker.

If not otherwise obvious, I skipped through conversations with Slayne, skipped the cutscenes upon entering his cave, and jigged south of Jaffar around the giant rock to avoid triggering a conversation with him.

If it helps, there is a message in a bottle just after fighting the Sunken Ones; it provides a treasure map when inspected in the inventory, which updates your sea chart.

Based off of my numbers above, I needed to approximately kill an additional 840 monsters. I did this on my second playthrough, after running everything else except Isle of the Dead. (So, I ran Maracai Bay, Trasure Isle, and most of Air Temple first.)

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